Bio-microelectromechanical system transducer and associated methods

Abstract

The invention discloses a bio-MEMS transducer comprising a cultured myotube and a piezoelectric microcantilever having the myotube attached thereto along a lengthwise extent of said microcantilever. The transducer may include an input/output processor operably connected with said piezoelectric microcantilever to process electrical signals received therefrom and to send electrical signals thereto. The invention may operate as a biosensor wherein the attached myotube contracts on contact with a sensed agent, the myotube contraction deflecting the microcantilever to generate a piezoelectric signal therefrom. The invention may also be used as a biosensor for quantitating physiologic response to an agent by measuring deflection of the cantilever caused by myotube contraction elicited by contact with the agent; and correlating the measurement to effectiveness of the sensed agent in causing a myotube physiologic response. The bio-transducer is a bioactuator when an applied electrical signal causes the piezoelectric microcantilever to deflect, thereby actuating the attached myotube.

Description

RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 12/661,323, which was filed on 15 Mar. 2010, which claims priority from provisional applications Ser. No. 61/159,851 which was filed on 13 Mar. 2009, and Ser. No. 61/259,715, which was filed on 10 Nov. 2009, the disclosures of which are incorporated herein by reference in their entirety.

STATEMENT OF GOVERNMENT RIGHTS

This invention was made with Government support under agency contract/grant no. R01 NS050452 awarded by the National Institutes of Health. The government has certain rights in this invention.

FIELD OF THE INVENTION

The present invention relates to the field of muscle physiology and, more particularly, to a bio-microelectromechanical system (MEMS) useful as a transducer in testing drugs and actuating muscle tissue.

BACKGROUND OF THE INVENTION

Microelectromechanical systems (MEMS) have received a great deal of attention in recent years due to their promise for miniaturizing systems for a variety of applications. One particularly interesting facet of MEMS technologies is the possibility of coupling solid state devices with biological components (Bio-MEMS) such as biomolecules, cells, and tissues for creating novel bioanalytical systems.

Bio-MEMS technologies present a unique opportunity to study fundamental biological processes at a level unrealized with previous methods. The capability to miniaturize analytical systems enables researchers to perform multiple experiments in parallel and with a high degree of control over experimental variables. This capacity allows a high throughput approach for studying a wide variety of problems in biology.

Skeletal muscles are highly differentiated organs whose primary function is to generate longitudinal force for locomotion. Anatomically, myotubes or myofibers are composed of densely packed proteins (myofibrils), mostly myosin and actin, organized into functional structures called sarcomeres. During force generation the distance between the interconnected sarcomeres decreases as myosin pulls on the actin filaments. The process is mediated by Ca²⁺ release from the sarcoplasmic reticulum and is known as the sliding filament theory of muscular contraction (Huxley 1975; Gordon et al. 2000). Adult skeletal muscle is composed of two distinct types of fibers, extrafusal and intrafusal. The extrafusal fibers are part of the force generating motor circuit while the intrafusal fibers form the muscle component of a stretch sensor. Extrafusal and intrafusal fibers differ morphologically, functionally, and by their neuronal innervation.

The general structure of the sarcomere is consistent among extrafusal muscle fibers. However, adult skeletal muscle expresses multiple isoforms of myosin heavy chain (MHC) protein. Each isoform exhibits distinct ATPase activity that alters the physiological properties of the sarcomere and the myofiber overall. MHC classes can be divided into three isoforms type I, type IIa, and type IIb (Walro and Kucera 1999).

Type I muscle fibers contact slowly relative to the other isoforms due to slow ATPase activity and are slow to fatigue due to high levels of mitochondrial enzymes that generate large amounts of ATP. They contain a large number of mitochondria and myoglobin which give them a distinctive red color and are, therefore, known as red fibers. These fibers rely on aerobic respiration for ATP regeneration and are responsible for sustained, tonic contraction. They typically maintain an intracellular calcium level above 100 nM, but below 300 nM (Olson and Williams 2000; Scott et al. 2001). In vivo evidence suggests that chronic long term stimulation of fast twitch muscle fibers like the tibialis anterior causes a switch to the slow MHC isoform (Termin and Pette 1992; Pette et al. 2002). The integral membrane protein phospholamban is expressed exclusively in type I fibers where it regulates the Ca²⁺ pump adding an additional level of contractile rate control (Pette and Staron 2001).

Type IIa myofibers can be considered an intermediate between fast and slow twitch fibers. These muscle fiber types also contain a high number of large mitochondria as well as increased myoglobin levels, which also gives them a red appearance. However, they are able to split ATP rapidly which gives the myotubes a high contractile velocity. They are resistant to fatigue because of their high capacity to regenerate ATP by oxidation, but not as resistant as Type I fibers (Scott et al. 2001). The Ca²⁺ binding protein parvalbumin is expressed exclusively in Type II fibers where it aids in muscle relaxation by removing Ca²30 from the cytoplasm of the myofiber (Pette and Staron 2001).

Type IIb fibers are known as white fibers due to their low levels of mitochondria and myoglobin. They also possess few blood capillaries and consequently rely on anaerobic respiration for ATP regeneration. Type IIb fibers contain a large amount of glycogen and split ATP very rapidly. These factors leave these muscle fibers prone to fatigue. Fast twitch glycolytic fibers (type IIb) are used for sudden bursts of contraction and are characterized by brief, high-amplitude Ca²⁺ transients and lower ambient Ca²⁺ levels (<50 nM) (Olson and Williams 2000; Scott et al. 2001). It has been previously determined that the increased thyroxine (T4) and triiodothyronine (T3) levels in hyperthyroid animal models results in a conversion to type II fibers while hypothyroidism results in conversions in the opposite direction (Caiozzo et al. 1992). In vivo data has also established that calcineurin will de-phosphorylate Nuclear Factor of Activated T-cells (NFATs) which will allow them to translocate from the cytosol to the nucleus. Once in the nucleus, they bind to promoters and enhancers that activate slow fiber type formation (Schiaffino and Serrano 2002). Cyclosporin is an inhibitor of the calcineurin signaling pathway (Schneider et al. 1999). Akt 1 induces type IIb fiber formation and can be activated by platelet derived growth factor (PDGF) (Izumiya et al. 2008).

Intrafusal fibers reside in specialized sensory structures called muscle spindles. Morphologically, muscle spindles consist of two to twelve intrafusal fibers that are distinct from the extrafusal fibers in both structure and function. These unique fibers can be categorized morphologically as nuclear bag1, nuclear bag2 and nuclear chain fibers based on the location of their nuclei (Matthews 1964; Kucera 1982b). In nuclear bag fibers, the nuclei are clustered in an enlarged central region, while in nuclear chain fibers, the nuclei are arranged in a single row localized in the equatorial region (Kucera 1982a; Kucera 1983). The spindle fibers are also unique in that morphological characteristics such as striation and myofibril density vary proportionately with distance from the center (Kucera and Dorovini-Zis 1979). In fact, they are heavily striated at their polar regions indicating the presence of contractile sarcomeric units, a feature that decreases moving equatorially until it is nearly absent. This feature plays an important role in the sensitivity to stretch seen in fibers and consequently the nerve terminals that innervate them. Due to their unique role in sensory perception, these fibers express a distinct protein called α cardiac-like MHC at their equatorial region.

One tissue of particular interest with respect to a variety of diseases is skeletal muscle. Diseases affect skeletal muscle in different ways. Some diseases, such as amyotrophic lateral sclerosis (ALS), affect the stimulating inputs from the neuromuscular junction. Other diseases affect the muscle directly such as muscular dystrophy and muscular atrophy, which cause deterioration of the muscles' ability to generate force. Thus, it is advantageous to have a system that allows the real-time interrogation of the physiological properties of muscle as well as the controlled addition of exogenous factors for comparative experimentation. However, it is first necessary to be able to apply the measurements to statistical analysis with regard to physiological factors such as peak stress generated, time to peak stress, the time needed for the muscle to relax to half of the peak stress, and the average rate of stress generation. All of these factors give information about the condition of the muscle and can be compared to published values.

The present study outlines a novel method for performing real-time quantitative measurements of the physiological properties of cultured skeletal muscle using a Bio-MEMS device. Stresses generated by myotubes were measured using a modified Stoney's equation, which quantifies stresses generated by a thin film on a microcantilever with known physical properties. By this method it has been shown that it is possible to quantitatively measure stress on microcantilevers that are in agreement with values previously published in the literature for cultured skeletal muscle. Furthermore, a method for selectively seeding and coculturing neuronal and muscle cells on these devices using microfluidic chambers was developed. By this method it was possible to create a model for studying neuromuscular junction development and function. This work validates the use of this system as a foundation for a high-throughput Bio-MEMS device.

SUMMARY OF THE INVENTION

With the foregoing in mind, the present invention advantageously provides a bio-MEMS transducer comprising a cultured myotube and a piezoelectric microcantilever having the myotube attached thereto along a lengthwise extent of said microcantilever.

In the invention, the bio-MEMS transducer may also further comprise an input/output processor operably connected with said piezoelectric microcantilever to process electrical signals received therefrom and to send electrical signals thereto. Optionally, the bio-MEMS transducer may also further include an optical sensor positioned to sense microcantilever deflection. The optical sensor preferably includes a laser aimed to reflect from said microcantilever and a photodetector positioned to detect a change in reflection angle of said laser.

The presently disclosed bio-MEMS transducer functions as a biosensor wherein the attached myotube contracts on contact with a sensed agent, the myotube contraction deflecting the microcantilever to generate a piezoelectric signal therefrom. In this embodiment, the sensed agent may be selected, without limitation, from metabolic inhibitors, nutritional supplements, therapeutic compounds or compositions, investigational drugs, and combinations thereof. For example, the disclosed bio-MEMS sensor may be used to investigate the effectiveness on muscle tissue of a new investigational drug and, more specifically, if the myotube were cultured from cardiac muscle, new drugs for cardiac use could be evaluated with this system.

Accordingly, the bio-MEMS transducer herein disclosed could be used in a method for quantitating physiologic response to an agent. This method comprises measuring deflection of the cantilever caused by myotube contraction elicited by contact with the agent and correlating the measurement to effectiveness of the sensed agent in causing a myotube physiologic response.

Conversely, the bio-MEMS transducer of the present invention could be employed as a bioactuator wherein an applied electrical signal causes the piezoelectric microcantilever to deflect, thereby actuating the attached myotube. There are several variations of this embodiment. For example, the bio-MEMS transducer of can act as a bioactuator wherein an applied electrical signal causes the piezoelectric microcantilever to deflect, thereby actuating the attached myotube. Also, the bio-MEMS transducer acts as a bioactuator wherein a neuron is synapsed to the myotube so as to apply a stimulus thereto, thereby actuating the myotube, the piezoelectric microcantilever being responsive to the actuation. Additionally, the bio-MEMS transducer acts as a bioactuator wherein a neuron is synapsed to the myotube so as to apply a stimulus thereto, thereby actuating the myotube, the optical sensor being responsive to the actuation. The bio-MEMS transducer of may also be employed as a bioactuator wherein a signal or stimulus is applied to the piezoelectric microcantilever to deflect, thereby actuating the attached myotube responsively sending a signal to activate an associated sensory neuron. The bio-MEMS transducer may also be employed in a prosthetic system wherein the cultured myotube is responsively associated with a severed neuron providing a stimulus thereto so as to actuate the cultured myotube to deflect the piezoelectric microcantilever.

Yet another aspect of the invention includes a method of quantitatively measuring the physiologic response of cultured skeletal muscle to a test agent. This method comprises fabricating the bio-MEMS by depositing isolated mammalian myocytes onto a non-biological organosilane substrate patterned into one or more relatively flexible microcantilevers. The deposited myocytes are cultured so as to promote their growth into myotubes generally aligned along a lengthwise extent of the one or more microcantilevers having a free end. The method continues by contacting the myotubes with a test agent, then monitoring the one or more microcantilevers so as to detect any deflexion thereof due to a myotube physiologic response to the test agent and numerically quantitating the deflexion detected in the free end of the one or more microcantilevers as an indicator of myotube physiologic response to the test agent.

The method optionally includes the microcantilevers being coated with amine-terminated alkylsilane (3-Trimethoxysilyl propyl) diethylenetriamine (DETA). Also optional in the method is that depositing may preferably comprise a density of approximately 500-800 isolated myocytes and wherein culturing extends for about 10-13 days. In the method, the test agent is preferably, but not exclusively, selected from metabolic inhibitors, nutritional supplements, therapeutic compositions, investigational drugs, and combinations thereof.

Monitoring in a non-piezoelectric embodiment of the invention best comprises a laser and a photodetector mounted on x-y-z-θ translators associated with a microscope and numerically quantitating comprises applying calculations to microcantilever deflexions detected, so as to numerically express deflection of the free end of the one or more microcantilevers.

Preferably, numerically quantitating in the method comprises applying Stoney's equation and associated calculations to microcantilever deflexions detected, so as to numerically express deflection of the free end of the one or more microcantilevers.

In this application it should be understood that the terms “cantilever” and “microcantilever” are used interchangeably and are intended to identify the same component of the invention. Other terms are intended to have their ordinary meaning in the art to which they apply, as known to those of skill therein, unless the context clearly indicates a different meaning. For example, the term “transducer” is used herein as known in the art for a device that converts one type of energy into another. This conversion could be to or from electrical, electromechanical, electromagnetic, photonic, photovoltaic, and other forms of energy. In the present case the transducer employs a piezoelectric microcantilever having a myotube attached thereto. When the myotube contracts, it bends the microcantilever generating a piezoelectric signal. Conversely, if an electric signal is applied to the piezoelectric microcantilever, it will bend in response to the applied electric signal and as a result of the bending it will at the same time actuate the myotube. Accordingly, the presently disclosed bio-MEMS transducer may be employed as a sensor or as an actuator.

BRIEF DESCRIPTION OF THE DRAWINGS

Some of the features, advantages, and benefits of the present invention having been stated, others will become apparent as the description proceeds when taken in conjunction with the accompanying drawings, presented for solely for exemplary purposes and not with intent to limit the invention thereto, and in which:

FIGS. 1A-B show a layout of microcantilever devices generated in AutoCAD, all units are shown in microns; FIG. 1A) the layout of a single die, the outer boundaries, delimited by dashed lines which formed connecting tabs allowing the die to be easily separated, were designed to be 14.8 mm×14.8 mm; FIG. 1B) close-up view of a partial microcantilever row showing microcantilever dimensions (737 mm×100 mm) and spacing between microcantilevers (300 mm);

FIG. 2 depicts a schematic representation of an AFM detection system as might be used in the present invention;

FIGS. 3A-3D show SEM and confocal microscopy measurements of microcantilever and tissue thickness, wherein FIG. 3A) is a low magnification view (50° takeoff angle) of silicon microcantilevers; FIG. 3B) shows high a magnification view (50° takeoff angle) of microcantilever showing the measured thickness; FIG. 3C) is a top-down view of cultured myotube taken by confocal microscopy; and FIG. 3D) shows a digitally reconstructed side-view of cultured myotube showing measured thickness;

FIG. 4 presents raw data versus calculated stress for cultured embryonic muscle;

FIG. 5 indicates main parameters for muscle characterization;

FIGS. 6A-B show stress variation with measured film thickness, wherein FIG. 6A) presents data recorded from a contracting myotube plotted as a function of time and measured film thickness; FIG. 6B) shows peak stress plotted as a function of film thickness; the arrow indicates calculated stress value for 10 mm film thickness;

FIG. 7 presents line graphs generated when contractile myotubes were exposed to the sodium channel agonist veratridine; myotubes were contracting synchronously with the 1 Hz stimulus when, at 84 seconds into the recording, veratridine was injected; after injection of the toxin, the muscle tissue contracted in a tetanic manner and thereafter lost the ability to contract further;

FIGS. 8A-8B illustrate contraction kinetics from muscle tissue cultured in NB4 media, wherein FIG. 8A) is raw data recorded from Bio-MEMS device showing TPT and ½RT, and FIG. 8B) shows stress values calculated using Stoney's equation;

FIGS. 9A-9B show a schematic diagram of optical (FIG. 9A) and piezoresistive detection (FIG. 9B) of cantilever beam bending;

FIG. 10A shows a perspective view of the microcantilevers of the present invention and FIG. 10B shows myotubes growing on the microcantilevers;

FIGS. 11A-E show myotubes immunostained with neonatal myosin heavy chain (N3.36); scale bar: 75 micro; FIG. 11A) phase picture of 2 myotubes shown by white arrows; FIG. 11B) both the myotubes shown in phase (FIG. 11A) have acetylcholine receptor clustering shown by alpha-bungarotoxin staining; FIG. 11C) only one myotube out of the two seen in FIG. 11A stained for N3.36, which is the neonatal, heavy chain antibody; FIG. 11D) double stained picture of the FIG. 11A with alpha-bungarotoxin and N3.36; FIG. 11E) representative current clamp trace;

FIGS. 12A-B show adult rat satellite cell myogenesis, (FIG. 12A) day 5 satellite cell myoblasts and (FIG. 12B) day 7 myotubes formed by satellite cell fusion;

FIGS. 13A-F show fetal skeletal muscle myotubes immunostained for adult MHC isoforms; (FIG. 13A) phase contrast image, (FIG. 13B) BF-F3 type IIb staining, (FIG. 13C) color composite, (FIG. 13D) phase contrast image, (FIG. 13E) BA-F8 type I staining, (FIG. 13F) color composite; scale bar=75 μm;

FIGS. 14A-E silicon based cantilevers and an AFM detection system, can be used to detect myotube contraction; FIG. 14A) simplified schematic of AFM-based detection system; FIG. 14B) phase contrast image of a myotube from post natal rat grown for 8 days on silicon cantilevers; FIG. 14C) fluorescence image of myotube stained for α-actin; arrow heads indicate the location of the myotube; FIG. 14D) experimental setup of detection system; FIG. 14E) oscilloscope reading of the voltage changes on the photodiode due to contractions of myotube utilizing periodic electrical field stimulation at 1 Hz; the top trace shows the timing of the stimulus trigger, while the lower trace shows the response on the photodiode (PD);

FIG. 15A shows the co-culture at day 9, 40×; a neuron with MN morphology sends out long axon toward a striated myotube as indicated by the red arrow; FIG. 15B shows striated myotubes can be found frequently in the coculture; the striations of the myotubes are indicated by the yellow arrows; FIG. 15C and FIG. 15D show hSKM on cantilevers, day 4 in serum-free growth medium;

FIGS. 16A-B illustrate schematics of piezo-cantilever construction; (FIG. 16A) micro fabrication of cantilevers with piezoelectric elements (left) and, as an alternative approach, with piezoresistive materials (right); gray lines indicate final position of cantilevers; gold electrodes and wiring are deposited in a first step; second follows the formation of piezoelectric (or piezoresistive) layers; top electrodes are Deposited in order to apply/read voltages across the zo piezo-layer for/during deflection; a final insulation layer protects all conducting elements from culturing conditions; (FIG. 16B) packaging of cantilever chips with printed circuit boards;

FIG. 17 provides a schematic diagram of a modified setup to address piezoelectric (or piezoresistive) microcantilevers with an MEA amplifier; and

FIGS. 18A-B show a top view of a multiplexed 24 well plate system (FIG. 18A) providing 16 microcantilevers per well; shown in an enlarged view (FIG. 18B); the size of the cartridge is the same as in a 96 well plate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, suitable methods and materials are described below. Any publications, patent applications, patents, or other references discussed or cited are incorporated herein by reference in their entirety. In case of conflict, the present specification, including any definitions, will control. In addition, the materials, methods and examples given are illustrative in nature only and not intended to be limiting. Accordingly, this invention may, however, be embodied in many different forms and should not be construed as limited to the illustrated embodiments set forth herein. Rather, these illustrated embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Other features and advantages of the invention will be apparent from the following detailed description, and from the claims.

Part I

Combining Biological Elements with Microcantilevers

Materials and Methods: Microcantilever Fabrication

The layout for the microcantilevers was generated using AutoCAD 2004 (FIG. 1). The patterns were written to chrome coated 4-5 inch soda-lime glass masks for front and back side photolithography. Microcantilevers were fabricated from 6 inch double-sided polished silicon-on-insulator (SOI) wafers with a 5 mm crystalline silicon layer (front side) and a 500 mm silicon dioxide layer (back side). The front side was primed with a 10 nm layer of hexamethyldisilazane (HMDS) to promote resist adhesion. A 5 mm layer of the photoresist AZ 5214 E (Clariant, Muttenz, Switzerland) was spun onto the device layer followed by softbake, alignment, exposure, and development. The device layer was etched using the deep reactive ion etch (DRIE) process at a rate of 2 mm/min. Resist was stripped and a 0.5 mm thick layer of silicon dioxide was deposited via Plasma Enhanced Chemical Vapor Deposition (PECVD) process to protect the device layer during subsequent processing. The wafer was then flipped over and was primed with a 100 Å layer of HMDS and spun with 4.15 mm layer of AZ 9245 photoresist (Clariant, Muttenz, Switzerland). Coating was followed by softbake, front-back alignment, development, and DRIE etch at 4 mm/min until the bulk of the back side had been etched through leaving only the buried native oxide layer. The devices were then immersed in a buffered HF dip to remove the buried native oxide layer as well as the protective silicon dioxide that had been deposited onto the device layer. Individual devices were separated by breaking connecting tabs that were incorporated into the device design. Microcantilever dimensions were measured using a JEOL 6400 scanning electron microscope (SEM) at a take-off angle of 50° off normal.

Prior to cell culture experiments the microcantilevers were coated with the amine-terminated alkylsilane (3-Trimethoxysilyl propyl) diethylenetriamine (DETA) according to previously established protocol. Prior to coating microcantilevers were cleaned using serial acid baths. Substrates were arranged in a porcelain coverslip holder (Thomas Scientific, Swedesboro, N.J.). The substrates were then immersed in a 1:1 (vol:vol) solution of methanol and concentrated HCl for at least 1 hour. This step removed surface contaminants. After 1 hour the substrates were rinsed 3× in diH₂O and transferred to a solution of concentrated sulfuric acid for at least 1 hour. This step oxidized the surface of the microcantilevers leaving a hydrophilic surface suitable for reaction of the silane derivatives. After at least one hour in sulfuric acid the substrates were washed 3× in diH₂O. The rinsed substrates were then boiled in diH₂O for 30 minutes. After boiling the samples were place in a 120° C. oven for at least 3 hours. The resulting surfaces were analyzed using contact angle goniometry and XPS to verify hydrophilicity of the surfaces (CA<5.0°) and the elemental composition of the surfaces respectively. Surfaces with a CA of less that 5.0° and an elemental carbon content of approximately 5.0% were considered suitable for derivitization.

After cleaning the microcantilevers, fresh distilled toluene was transferred into a Pyrex bottle that had been dried in an 120° C. oven to dry off excess surface water. Dry nitrogen was used to replace the air in the remaining volume of the bottle to minimize free oxygen. The bottle was sealed and placed in the antechamber of an MBraun glovebox, which was evacuated and refilled with dry nitrogen 3 times. The toluene was transferred into the main chamber. DETA was added to the toluene to a final concentration of 0.1% (vol:vol). The DETA-toluene solution was removed from the glove box and transferred to a pyrex beaker and the samples were immersed in the solution. To drive the reaction forward the solution was gently heated to no more than 65° C. Optimal reaction time was analyzed for these conditions by incubating the samples 10, 20, and 30 minutes. After reaction with DETA the samples were allowed to cool to room temperature, washed 3 time with dry tolune and heated too 65° C. for 30 more minutes. The resulting samples were analyzed by XPS and contact angle goniometry.

Cell Culture

Skeletal muscle was dissected from the hind limb thighs of a rat fetus at embryonic day 18 (Charles River Laboratories, Wilmington, Mass.) according to a previously published protocol with some modification. Tissue samples were collected in a sterile 15-ml centrifuge tube containing 1 ml of calcium and magnesium free phosphate buffered saline (PBS). Tissue samples were enzymatically disassociated using 3 ml of 0.05% of trypsin-EDTA (Invitrogen, Carlsbad, Calif.) solution for 60 min in a 37° C. water bath with agitation of 100 rpm. After 60 min, the trypsin solution was removed and 6 ml of L15 media (Invitrogen, Carlsbad, Calif.) containing 10% fetal bovine serum (FBS) was added to terminate the trypsin action. The tissue was then mechanically triturated using a sterile narrow bore Pasteur pipette, allowed to settle for 3 min, and transferred to a 15-ml centrifuge tube. This was repeated three times. The dissociated tissue was then centrifuged at 300 g for 10 minutes at 4° C. on 6 ml of a 4% (wt/vol) cushion of bovine serum albumin (BSA). The pellet was resuspended in 10 ml L15+10% FBS and plated in uncoated 100-mm Petri dishes for 20-30 min depending on the amount of tissue, to allow contaminating fibroblasts to settle out. After 20-30 minutes the supernatant was layered on 6 ml of a 4% BSA cushion, and centrifuged at 300 g for 10 min at 4° C. The pellet was resuspended in 1.5 ml of medium.

Purified myocytes were plated at a density of 500-800 cells per square millimeter onto the microcantilevers. Myocytes were allowed to attach for 1 hour after which time 3 ml of culture medium (Neurobasal media containing B-27 [Invitrogen, Carlsbad, Calif.], Glutamax [Invitrogen, Carlsbad, Calif.], and Pencillin/Streptavidin) was added. Cultures were maintained in a 5% CO₂ incubator (relative humidity, 85%). Culture medium was exchanged every 4 days. Microcantilever/myocyte constructs were allowed to culture for 10-13 days. During this time myocytes fuse into functional myotubes capable of generating contractile stresses sufficient to deflect the microcantilever. These cultures were used in experiments for validating the use of Stoney's equations equation for calculating contractile stress of the myotubes.

Detection System

A detection system similar to those used in atomic force microscopes (AFM) was designed for measuring deflection of the microcantilevers during myotube contraction and is illustrated in FIG. 2. The entire system was assembled around an upright Olympus BX51WI electrophysiology microscope (Olympus Inc., Center Valley, Pa.). The detection system consisted of a class 2 red photodiode laser (Newport, Irvine, Calif.), a stimulation chamber, a 4-quadrant photodetector (Noah Industries, Melbourne, Fla.), and a computer with pClamp 10.0 data acquisition software (Molecular Devices, Union City, Calif.). The laser and photodetector (PD) were mounted on x-y-z-θ translators (Newport, Irvine, Calif.) which were mounted on the underside of the microscope stage. The stimulation chamber was fabricated from 6 mm thick polycarbonate sheet. An approximately 15 mm×15 mm square chamber was milled out of the sheet and fitted with silver wires (0.015 inch diameter) for field stimulation. The silver wires were mounted parallel to each other with a separation of 15 mm. The bottom of the chamber was sealed using a 22 mm×22 mm glass coverslip. This created a transparent base through which the laser beam could easily pass. The silver wires were connected to an external pulse generator (A-M systems, Sequim, Wash.) capable of producing field stimulation pulses of varying intensity, frequency, and waveform. Both the pulse generator and PD were connected to an Axon Instruments series 1440 digitizer (Molecular Devices, Union City, Calif.) which was interfaced with the computer.

System Calibration

The AFM system was calibrated using a modified version of the optical lever method. A bare microcantilever die, without cells, was placed in the stimulation chamber. The laser was focused on one of the microcantilevers and the PD was adjusted so that the laser fell on the diode surface. Using a digital volt meter to monitor the output voltage, the PD was adjusted so that the voltage being read was less than −7 volts. The PD was then moved vertically in 5 mm increments and the voltage recorded at each position. The results were plotted in Excel and a linear regression line was fitted to the linear region of the calibration curve, which was the region between −5 and 5 volts. The slope of this region was the detector sensitivity (ydetector). This value was used to calculate the angle, θ, of the deflection at the end of the microcantilever using the equation:

$\begin{array}{cc}\theta =\frac{y_{\mathrm{measured}}}{2\cos \left(\phi \right)l\times y_{\mathrm{detector}}}& \mathrm{Equation}1\end{array}$

where, y measured is the voltage measured from the PD, φ, is the angle of the detector to normal, and l, is the path length of the reflected laser beam.

Stress Calculation

The stress exerted by a myotube attached along its length to a microcantilever can be estimated by considering the system as a microcantilever bimorph and using Stoney's equation, which relates the stress in a bimorph system (film on substrate) to curvature of the substrate and the mechanical properties and thicknesses of the substrate and adherent film layer. The film stress, sfilm, is:

$\begin{array}{cc}{\sigma }_{\mathrm{film}}=\frac{1}{6{\mathrm{Rt}}_{\mathrm{film}}}\left[\frac{E_{\mathrm{beam}}{t}_{\mathrm{beam}}^3}{\left(1-v_{\mathrm{beam}}\right)\left(t_{\mathrm{beam}}+t_{\mathrm{film}}\right)}+\frac{E_{\mathrm{film}}{t}_{\mathrm{film}}^3}{\left(1-v_{\mathrm{film}}\right)\left(t_{\mathrm{beam}}+t_{\mathrm{film}}\right)}\right]& \mathrm{Equation}2\end{array}$

where Ebeam and vbeam are the microcantilever material modulus (130 GPa) and Poisson's ratio (0.28), respectively, tbeam is the microcantilever thickness, tfilm is the myotube thickness, R is the effective radius of curvature of the beam caused by the stress in the myotube layer, σ_film.

Many applications of Stoney's formula, most recently for studies of deposited and adsorbed films on thin substrates or microcantilevers, neglect the second term in the brackets because the films are much thinner than the substrate. In the present Bio-MEMS system, this assumption is not satisfied (t_film ˜10 μm compared to the microcantilever thickness, t_beam>>5 μm). However, for the system we also neglect this term because the modulus of the myotube cells comprising the film on the microcantilever are expected to be in the kPa range, at least 6 orders of magnitude lower than the modulus of the beam substrate Si (130 GPa). Thus we write:

$\begin{array}{cc}{\sigma }_{\mathrm{film}}\approx \frac{E_{\mathrm{beam}}\left(t_{\mathrm{beam}}^3\right)}{6\left(1-v_{\mathrm{beam}}\right)t_{\mathrm{film}}\left(t_{\mathrm{film}}+t_{\mathrm{beam}}\right)}\frac{1}{R}& \mathrm{Equation}3\end{array}$

The radius of curvature of the microcantilever during contraction was calculated using the raw voltage data collected from the PD. This was done taking into account the geometry of the system (path length of the reflected laser, sensitivity of the detector, etc.). From the raw data the change in angle of the end of the microcantilever, θ, was calculated using equation 1. Using θ it was then possible to calculate the deflection of the free end of the microcantilever, d, by the relation from Butt et al:

$\begin{array}{cc}\delta =\frac{2\theta L}{3}& \mathrm{Equation}4\end{array}$

where, L is the length of the microcantilever. Experimentally, 1/R is estimated using the measured beam deflection and the geometric approximation from Ratieri et al.:

$\begin{array}{cc}\frac{1}{R}\approx \frac{3\delta }{2L^2}& \mathrm{Equation}5\end{array}$

From FIG. 2 it can be seen that tensile or compressive stress in the myotube film will result in an upward or downward vertical deflection of the microcantilever beam. Measured deflections from the photodiode detector will be reported as positive and negative deflection δ, respectively. Since the myotube film is grown on the top face of the microcantilever array, deflections due to tensile (positive values) or compressive (negative values) stresses in the film are consistent with standard conventions. All calculations were performed using Matlab.

Immunostaining and Confocal Microscopy

After AFM measurements the tissue samples were washed 3× with PBS, then fixed for 15 minutes in 4% (vol/vol) paraformaldehyde at room temperature. Tissues were permeabilized and blocked in a single step using a solution of 0.1% Triton-X100 in PBS, with 10% donkey serum. Blocking and permeabilization were allowed to proceed for 1-2 hours. Afterward, the samples were washed 3× in PBS and incubated with a mouse anti-myosin heavy chain primary antibody (Developmental Studies Hybridoma Bank, Iowa City, Iowa) overnight at 20° C. Following incubation with the primary antibody, the tissues were washed 3× with PBS and incubated with a donkey anti-mouse secondary conjugated with Alexa Fluor 594 (Invitrogen, Carlsbad, Calif.) at room temperature for 1-2 hours. The final stained samples were washed again with PBS and imaged under PBS using confocal microscopy.

Myotube thickness was measured by optical sectioning with a Perkin Elmer Ultraview spinning disc confocal microscope (Perkin Elmer, Waltham, Mass.) under a 40× water immersion lens. The 40× lens was mounted on a piezoelectric z-step motor with a minimum step size of 0.4 mm and a total travel length of 60 mm. Images were collected in 0.5 mm steps from the surface of the microcantilever to the top of the tissue. The “z-stack” of images was reconstructed using a 3-D rendering program provided with the microscope. The thickness of the myotube was then measured using the reconstructed image and an internal reference scale.

Exogenous Factors Added to Muscle Culture

In order to demonstrate the usefulness of this device for studying the biology of muscle development and function, experiments were conducted using exogenously applied factors to elicit a measurably different response of the muscle compared to control conditions. The sodium channel agonist veratridine was added to normally cultured myotube on microcantilevers and the response was measure with the AFM detection system. After 10 days of culture the myotube/microcantilever constructs were place in the AFM detection system and stimulated with a 1 Hz pulse to elicit synchronous, detectable contraction. Upon confirmation of synchronous contraction veratridine was added to a final concentration of 5 mM, and the resulting contractile behavior recorded.

Cultures were also performed in order to enhance the contractile capacity of the myotubes. The culture medium NbActiv4 was used in lieu of the Neurobasal/B27 formulation used for control cultures. Microcantilevers were seeded normally and cultured under conditions identical to those previously stated. After 10-13 days microcantilever/myotube constructs were placed in the AFM detection system and stimulated with a 1 Hz pulse train. The calculated values were then compared to previous experiments and published literature.

Results and Discussion

Characterization of Microcantilevers

When using Stoney's equation to estimate film stress on microcantilevers it is important to have precise knowledge of the thickness of both the beam and the film. FIG. 3 shows representative SEM micrographs of the microcantilevers used for the experiments. The microcantilevers were measured to have a mean length and width of 755+/−3 mm and 109+/−1 mm. As shown in FIG. 3B the mean thickness of the microcantilevers was measured to be 5.27+/−0.07 mm. Given these values one can expect a ˜4% error in the stress estimation from experiment to experiment due to variation in beam thickness.

The spring constant of the microcantilevers was calculated theoretically and measured experimentally. The calculated spring constant, 1.21 N/m, was determined from the measured dimensions and Young's modulus for crystalline silicon, using formula for the spring constant of a rectangular microcantilever. The spring constant was determined experimentally using the method of Sader et al. 102. In short the resonant frequency was measured via ring-down experiment, and the resulting data processed by the spectrum analysis routines in the pClamp software. The resonant frequency in air was determined to be 88.5 kHz. Corrected for damping by air, the resonant frequency of the microcantilevers was found to be 88.7 kHz. This value was then applied to Sader's equation for calculating the microcantilever spring constant, which was found to be 1.26 N/m. Due to the high resonance frequency of the microcantilevers, it is expected that the resulting data reflects only the behavior of the myotube contraction as the response time of the microcantilevers is on the order of microseconds, whereas the time scale of the muscle contraction is on the order of milliseconds.

Myotube Culture

After plating the dissociated myocytes on the microcantilevers, the Bio-MEMS constructs were allowed to culture for 10-13 days during which time the myocytes fused into functional myotubes. During fusion and differentiation, the myotubes spontaneously orient along the long axis of the microcantilever, facilitating bending of the microcantilever. It should be noted, however, that the orientation of the myotubes was not always directly parallel to the long axis of the microcantilever. This configuration resulted in some torsional bending, and hence a possible underestimation of the total contractile stress. Typically the coverage of myotubes on microcantilevers was greater than 95%. Occasionally, tissue coverage was less due to tissue processing, suboptimal surface modification, and other systematic errors. Only robust cultures with morphologically normal looking myotubes were used for AFM experiments. FIG. 3c shows a confocal microscope image of a section of a representative myotube cultured for 13 days on a DETA modified microcantilever (not visible). FIG. 3a shows the top down projection of the z-stack in the plane of the microcantilever. The data from the z-stack of images were reconstructed into a 3-dimensional representation of the myotube geometry. FIG. 3b is a side view showing the thickness of the myotube along a section of the microcantilever. The mean thickness of the myotube was ˜10 mm. Due to the morphology of the myotube, however, the thickness is not necessarily uniform throughout the length of the microcantilever. The thickness has been measured to range between 5 mm to 15 mm on an individual microcantilever. This variation in film thickness throughout the tissue can potentially lead to discrepancies between true and calculated stress values. In this study we used the average value of 10 mm for calculations. The effect of the thickness variation on the calculated stress is considered in a later section.

Stress Calculation

FIG. 4 shows both the raw voltage data from the PD and the resulting stress calculated using the Stoney's equation. FIG. 4a shows the raw data collected in free-run mode from myotubes cultured for 13 days and stimulated with a 5 volt DC pulse at a frequency of 1 Hz. As shown previously, this allowed selective stimulation of the myotubes to actuate the microcantilevers. Each trigger pulse, FIG. 4c, corresponds precisely with the onset of a myotube contraction. The myotubes responded to the stimulation in a frequency dependent manner; increasing or decreasing the stimulation frequency would result in a corresponding change in the frequency of myotube contraction. As with previously published results, stimulation at or above a frequency of 10 Hz induced a state of fused tetanus. FIG. 4b-c shows the resulting Stoney's calculation using the raw data. The stresses calculated from this data set range between 1.1 kPa and 1.4 kPa. These values are in excellent agreement with previously published literature for cultured skeletal muscle, which report average peak twitch stress values of 2.9 kPa (reported as specific peak twitch force in units of kN/m2), but less than 1% those expected for adult muscle, ˜300 kPA. This is not surprising due to the fact that the tissue used in this study was collected from embryonic stage rat pups and cultured in vitro for only 13 days after dissection. It is possible that the culture conditions, as published here, are not sufficient for the development of myotubes with adult phenotype. Similar observations were made by Dennis and Kosnik 103 for cultured adult rat myoids noting the possibility of developmental arrest in culture preventing the development of adult isoforms of myosin.

To further characterize the myotubes, three other parameters were analyzed: the time to peak twitch stress (TPT) was measured, which is the time required to reach stress from the onset of contraction, time to half relaxation (½RT), which is the time require to relax to 50% of peak tension, and the average stress generation (ds/dt), which is the slope of the force curve between 20% and 80% of peak tension (FIG. 5). As shown in Table 1, the resulting average contractile stress for these data is ˜1.2 kPa which is in agreement with previously stated results. The calculated values for TPT, and ½RT were significantly longer than those published for cultured muscle by Dennis and Kosnik 103 and for adult rat muscle as published by Close. The average TPT for the cultured myotubes was 236.8±26.1 ms. This value is considerably slower than that of 69.3±9.4 ms published by Dennis for cultured rat myoids as well as values of 65.0±3.8 ms and 36.0±2.3 ms, for neonatal and adult rat respectively, published by Close. The ½RT values for cultured myotubes were also prolonged compared to those reported by Dennis and Kosnik and Close. The average ½RT for the data presented in Table 1 was measured to be 233.6±23.8 ms. Dennis reported ½ART values of 116.4±19.4 ms for myoids, while Close reported values of 70.0±4.9 ms for neonatal muscle and 48.0±3.4 ms for adult muscle. It can be concluded from these data that the myotubes cultured in the Bio-MEMS system, while exhibiting contractile stress magnitudes comparable with those previously published for cultured rat muscle, show evidence of a more embryonic phenotype with regard to other important physiological parameters. This is further reinforced by previously published results showing staining of similarly cultured myotubes for embryonic myosin heavy chain, but not adult or fetal isoforms.

Variation in Stress Calculation Due to Film Thickness

As previously stated the thickness of the myotube film on the microcantilever has been measured to vary between 5 to 15 mm (FIG. 3). FIG. 6 shows the variation in calculated stress due to film thickness. FIG. 6a is a plot of the variation in calculated stress using the average of 11 contractions versus the film thicknesses used in the Stoney's calculation ranging from 5 to 15 mm. It is clear from this graph that there is a significant variation in the calculated stress due to the measured film thickness. Error! Reference source not found.b shows a plot of the calculated peak contractile stress vs. film thickness. In this plot it can be seen that the stress values range from ˜0.5 kPa to ˜3.2 kPa over the selected film thickness values. It is interesting to note that the Stoney's calculation is particularly sensitive to variations in the film thickness in the range encountered here. Below 5 mm the stress values increase exponentially. Above 15 mm the change in stress due to film thickness slows considerably. This reinforces the need for accurate measurements of the myotube thickness and standardization of the culture methods to minimize variations of the same. It should be noted, however, that even though there is obviously significant variation in calculated stress these values are still within the range of those published by Dennis and Kosnik (0.9 kN/m2 to 5.0 kN/m2). These results validate this approach as a method for measuring contractile stress generated by cultured skeletal muscle in a Bio-MEMS device.

Action Of Exogenous Modulators Of Myotube Function

The Sodium Channel Agonist Veratridine

Given the ability of this method for quantifying muscle contraction force and dynamics in real-time, it is ideal for studying the effect of exogenously applied factors on muscle physiology. One such example was the addition of the toxin veratridine to the stimulation chamber during electrical stimulation. Veratridine is an agonist that causes the persistent opening of voltage-gated sodium channels. Normally, upon depolarization of the cell membrane from electrical stimulation, voltage-gated sodium channels open allowing an influx of sodium ions into the cytoplasm which further depolarizes the sarcoplasmic reticulum causing calcium release and contraction. After a certain refractory period the voltage-gated sodium channels close and the resting membrane potential is restored. Veratridine binds to the voltage-gated sodium channels causing a persistent release of sodium into the cytoplasm followed by contraction, and if it is not removed cell death. FIG. 7 shows the recording of contracting skeletal muscle before and after addition of veratridine. Before addition the muscle was contracting normally in synchronization with the one Hz stimulus. At 84 seconds veratridine was injected into the stimulation chamber and allowed to diffuse to the tissue. As seen in FIG. 7 upon injection of the veratridine the muscle began to contract in an asynchronous, tetanic, manner with a peak stress far beyond those of the synchronized contractions. After the initial tetanic contraction, the muscle then lost the ability to further contract and the stress exerted on the microcantilever returned to baseline. This is the reaction that is expected upon exposure to this toxin.

Growth of Myotube in NbActiv4 to Enhance Muscle Contractility

As stated in previous sections, the contactile phenotype of the muscle cultured in this system was of an embryonic nature. For this device to serve as a model system for the study of normal muscle it is necessary to be able to culture muscle of a more adult phenotype. In order to do this it is necessary to supply additional factors that promote the development of more mature contractile properties in the myotubes. The culture medium NbActiv4 is a proprietary formulation based on Neurobasal medium and the growth factor cocktail B27104. NbActiv4 contains three additional growth factors (creatine, cholesterol, and estrogen) that have been shown to produce an eight-fold increase in spike activity in cultured neurons. However, these extra growth factors are also important for the development of the contractile mechanism of skeletal muscle. For this reason we cultured embryonic skeletal myotubes grown on silicon microcantilevers in NbActiv4 to quantify the changes in myotube development due to the added growth factors. FIG. 8 shows representative contraction data for myotube. FIG. 8a shows the raw data recorded by AFM for NbActiv4 cultured muscle. Here it can be seen that the TPT is measured to 172.1 ms and the ½ RT 175.7 ms.

Table 2 shows comparison of NbActiv4 cultured muscle with previously published results as well as myotubes cultured in Neurobasal/B27. It can be seen that the addition of NbActiv4 enhances the contractile properties of the myotubes significantly. Most notably the contractile stress generated by NbActiv4 myotubes, 3.2 kPa, is approximately 3 fold higher than those cultured in Neurobasal/B27, 1.1 kPa. Although this value is still much less than the stress generated by adult muscle, it is comparable to that published by Dennis and Kosnik 103. Also, TPT and ½RT values for NbActiv4 myotubes have decreased significantly compared to muscle cultured in Neurobasal/B27. This decrease in contraction time demonstrates that the myotubes are being pushed down a path towards a more mature phenotype, and developing fast-twitch isoforms of myosin, while increasing the speed of contraction. Furthermore, the increase in average stress generation (ds/dt) by almost five fold reinforces the argument that the contractile apparatus of myotubes grown in NbActiv4 is more mature and capable of greater stress generation.

Part II

Electromechanical Components of Other Preferred Embodiments

Having demonstrated above that cultured myotubes may be grown directly on MEMS devices such as microcantilevers, we now turn to the strictly electromechanical features of another preferred embodiment of the invention.

Since biological systems are considered unpredictable, we have herein provided data showing that our bio-MEMS devices work and are, thus, useful as a system for measuring the effect of additives on muscle contractility. These additives could be test compounds, for example, investigational drugs and the like. Muscle contractility causes deflection in the microcantilevers and can be measured by the optical laser device described. However, there are other approaches to measuring microcantilever deflection.

For example, if the microcantilevers are fabricated of a piezoelectric material or composite, deflection caused by myotube contractility generates a measurable piezoelectricity.

Since we have shown that cultured myocites may be grown into functional myotubes attached to microcantilevers we now consider the purely electromechanical features of another preferred embodiment of the invention. As with any other electrical and/or mechanical inventions, electromechanical structures function in predictable ways and these aspects of the present invention may be constructively reduced to practice without need of experimental proof, as required in a biological system.

In fact, piezoelectric microcantilever fabrication and function have been described by Choudhury et al., “A piezoresistive microcantilever array for surface stress measurement: curvature model and fabrication”, in J. Micromech. Microeng., 17 (2007) 2065-2076; and by Datar et al., “Cantilever Sensors: Nanomechanical Tools for Diagnostics”, in MRS Bulletin, Vol. 34, June 2009, pp. 449-454. Both of these publications are incorporated herein by reference in their entireties.

Electromechanical Aspects of the Present Invention

Electrically stimulating a muscle cell or a myotube causes it to contract. This can be monitored using electrophysiology or by a direct force measurement. A benefit of direct force deduction is that artifacts due to the intracellular patch-clamp recordings are avoided and a closer approximation to in vivo conditions is achieved (Eisen and Swash 2001). In the present invention, because the cell is bound to the surface of the cantilever, the cantilever will also bend or deflect upon stimulation of the bound muscle cell or myotube. Examples of a myotube's ability to deform a silicon beam have been reported (Xi et al. 2005; Wilson et al. 2007). By measuring the deflection of a cantilever, the force exerted by a cell can be calculated. Microfabricated cantilevers are used routinely in atomic force microscopy (AFM) to measure small forces on the picoNewton to microNewton scale. The detector measures the cantilever's displacement using Hooke's Law, F=−kz, where z is the cantilever displacement. The displacement can be measured by using an optical detector where the light beam from a diode laser is reflected off the backside of the cantilever onto a position sensitive detector.

Microfabricated cantilevers have spring constants in the 0.01 to 100 N/m range and displacements as small as 0.01 nm can be measured with the optical detection scheme (FIG. 9). A muscle cell attached to a cantilever surface will have a similar effect to placing a coating or adsorbate on the cantilever surface and can be modeled as a bimorph using Stoney's relationship (Stoney 1909). In a bimorph cantilever system, a stress in the attached film (e.g. a muscle contraction) results in the beam bending; the stress in the film is balanced by the stiffness of the cantilever. Biosensing applications in general utilizing the cantilever system are reviewed by Raiteri (Raiteri et al. 2001). The forces exerted by contracting muscle cells and muscle fibers are of the order of μN but could be as high as several hundred μN for bundles (Yasuda et al. 2001). Hence, large cantilever deflections are anticipated even after accounting for geometric and mass-loading effects associated with attaching the cell to the cantilever and viscosity damping effects caused by operating in an aqueous environment. Also by growing myofibrils on a cantilever a 3D environment is established and since the cantilevers can bend in response to spontaneous contractions, myotubes do not come off these surfaces, so long-term cultures can be maintained (FIG. 10).

Force Measurements from Calculations of Myofibril/Cantilever Construct Stress Values

The stress, or force, exerted by a myotube attached along its length to a cantilever can be estimated by considering the system as a cantilever bimorph and using Stoney's equation (Stoney 1909). Stoney's equation relates the stress in a bimorph system (film on substrate) to curvature of the substrate and the mechanical properties and thicknesses of the substrate and adherent film layer. We can calculate the film stress, σ_film according to the following formula:

$\begin{array}{cc}{\sigma }_{\mathrm{film}}=\frac{1}{6{\mathrm{Rt}}_{\mathrm{film}}}\left[\frac{E_{\mathrm{beam}}{t}_{\mathrm{beam}}^3}{\left(1-v_{\mathrm{beam}}\right)\left(t_{\mathrm{beam}}+t_{\mathrm{film}}\right)}+\frac{E_{\mathrm{film}}{t}_{\mathrm{film}}^3}{\left(1-v_{\mathrm{film}}\right)\left(t_{\mathrm{beam}}+t_{\mathrm{film}}\right)}\right]& \left(1\right)\end{array}$
where E_beam and v_beam are the cantilever material modulus (130 GPa) and Poisson's ratio (0.28), respectively, t_beam is the cantilever thickness, t_film is the myotube thickness, R is the effective radius of curvature of the beam caused by the stress in the myotube layer, σ_film.

Many applications of Stoney's formula, most recently for studies of deposited and adsorbed films on thin substrates (Sander et al. 1995) or cantilevers (Butt 1996; Moulard et al. 1998; Peterson et al. 1999), neglect the second term in the brackets because the films are much thinner than the substrate. In the present Bio-MEMS system, this assumption is not satisfied (t_film ˜10 μm compared to the cantilever thickness, t_beam≈5 μm). However, for the system we also neglect this term because the modulus of the myotube cells comprising the film on the cantilever are expected to be in the kPa range, at least 6 orders of magnitude lower than the modulus of the beam substrate Si (130 GPa). Thus we write:

$\begin{array}{cc}{\sigma }_{\mathrm{film}}\approx \frac{E_{\mathrm{beam}}\left(t_{\mathrm{beam}}^3\right)}{6\left(1-v_{\mathrm{beam}}\right)t_{\mathrm{film}}\left(t_{\mathrm{film}}+t_{\mathrm{beam}}\right)}\frac{1}{R}& \left(2\right)\end{array}$

The radius of curvature of the cantilever during contraction can be calculated using the raw voltage data collected from a photodiode. This is done by taking into account the geometry of the system (path length of the reflected laser, sensitivity of the detector to laser spot position, etc.) (Meyer and Nabil 1988; Alexander et al. 1989). From the raw data the change in angle of the end of the cantilever, θ, is calculated. Using θ it is then possible to calculate the deflection of the free end of the cantilever, δ, which is then applied to equation 3 to calculate the radius of curvature, R.

Experimentally, 1/R is estimated using the measured beam deflection and the geometric approximation

$\begin{array}{cc}\frac{1}{R}\approx \frac{3\delta }{2L^2}& \left(3\right)\end{array}$
where L is the length of the cantilever and δ is the deflection of the free end of the cantilever (Butt 1996). From FIG. 9, tensile or compressive stress in the myotube film results in an upward or downward vertical deflection of the cantilever beam. Measured deflections from the photodiode detector are reported as positive and negative deflection δ, respectively. Since the myotube film is grown on the top face of the cantilever array, deflections due to tensile (positive values) or compressive (negative values) stresses in the film are consistent with standard conventions (Müller and Saúl 2004) and give a direct readout of the force exerted in the system.
Piezoelectric Devices

Piezoelectricity is the ability of certain materials (crystals and certain ceramics) to generate an electric potential in response to applied mechanical stress (Holler et al. 2007). The piezoelectric effect is used in various sensors to measure stresses or geometrical deformations in various mechanical devices. The reverse piezoelectric effect turns piezoelectric materials into actuators, when an external voltage is applied to the crystal (King et al. 2000). Piezoelectric materials are e.g. quartz, bone, sodium tungstate, zinc oxide, or lead zirconate titanate (PZT) (Lou 2009). A similar effect is the piezoresistive phenomenon. When subjected to mechanical stress, these materials change their resistivity (Mutyala et al. 2009).

Our current cantilever system, designed for force measurements of contracting muscle cells uses laser optics as a readout system (Das et al. 2007). Piezoresistive and piezoelectric approaches are the most widely applied techniques for measuring stress applied on microcantilevers (Waggoner and Craighead 2007). The advantage is that the mechanical device and the read out electronics can be implemented in the same integrated circuit as in FIG. 9. Replacing the optical readout with piezoelements would reduce the size and complexity of our current cantilever system.

Hybrid System Allows Functional Integration of Cultured Myotubes with a MEMs Device and Detection of Myotube Contraction.

We have demonstrated methodology to create more fully differentiated muscle fibers that develop in a common medium that can also be used in the co-culture of motoneurons. These myotubes can be cultured out to 90 days on DETA ((Das et al. 2009), see paper in Appendix) and as shown in FIG. 11 are electrically active. We have fully developed the technology for the analysis of the functional myotubes on cantilevers. This work has been published as a Technical Note in Lab-on-a-Chip (Wilson et al. 2007). We have also been successful in preliminary attempts to fully differentiate embryonic rat myofibrils into adult isoforms as well as isolate satellite cells from adult rat muscle to form myofibrils in our defined, serum-free culture system as described below.

Growth of Myotubes in NbActiv4 to Enhance Muscle Contractility

For this device to serve as a model system for the study of normal muscle it is necessary to culture muscle of a more adult phenotype. Supplying additional factors that promote the development of more mature contractile properties in the myotubes is beneficial, as shown in FIG. 13. This would demonstrate that the system could be used to evaluate compounds that enhance myofibril development and functionality. The culture medium NbActiv4 is a proprietary formulation based on Neurobasal medium and the growth factor cocktail B27 (Brewer et al. 2008). NbActiv4 contains three additional growth factors (creatine, cholesterol, and estrogen) that have been shown to produce an eight-fold increase in spike activity in cultured neurons. However, these extra growth factors are also of importance in the development of the contractile mechanism of skeletal muscle. For this reason embryonic skeletal myotubes grown on silicon microcantilevers were cultured in NbActiv4 to quantify the changes in myotube development due to the added growth factors. FIG. 8A shows the raw data and calculated stress averaged from 11 myotube contractions (FIG. 8B) recorded by the photodiode for muscle cultured in NbActiv4. The resulting average contractile stress for these data is ˜1.1 kPa, which is in agreement with previously stated results. To further characterize the myotubes, two other parameters were analyzed: the time to peak twitch stress (TPT) was measured, which is the time required to reach peak stress from the onset of contraction, and time to half relaxation (½ RT), which is the time required to relax to 50% of peak tension. Here it can be seen that the TPT is measured to 172.1 ms and the ½ RT 175.7 ms.

Table 1 shows a comparison of the NbActiv4 cultured muscle with previously published results as well as myotubes cultured in Neurobasal/B27. It can be seen that the addition of NbActiv4 enhances the contractile properties of the myotubes significantly. Most notably the contractile stress generated by NbActiv4 myotubes, 3.2 kPa, is approximately 3 fold higher than those cultured in Neurobasal/B27, 1.1 kPa. Although this value is still much less than the stress generated by adult muscle, it is comparable to that published by Dennis et al (Dennis and Kosnik 2000). Also, TPT and ½ RT values for NbActiv4 myotubes have decreased significantly compared to muscle cultured in Neurobasal/B27. This decrease in contraction time demonstrates that the myotubes are being pushed down a path towards a more mature phenotype, and developing fast-twitch isoforms of myosin, while increasing the speed of contraction. Furthermore, the increase in average stress generation (dσ/dt) by almost five fold reinforces the argument that the contractile apparatus of myotubes grown in NbActiv4 is more mature and capable of greater stress generation, and validates the use of the myofibril/cantilever system as useful for analyzing temporal changes in myofibril function in response to drug candidates.

We have also shown that satellite cells from adult human muscle (hSKM) can be cultured in our defined, serum-free system and are functionally active. FIG. 15 shows a co-culture of human derived myofibrils and human motoneurons with classical striations indicative of sarcomere formation and mature myofibrils, as well as muscle only culture on the cantilevers in the serum-free media system.

Fabrication of the Microcantilever Hybrid System Chip.

The cantilevers are microfabricated out of silicon or silicon nitride, and prepared using DETA SAM modifications of the surface for cell attachment. The designs for the cantilevers are generated using AutoCAD 2004. Once designs are completed the AutoCAD file is used to create the photomask for device fabrication. The photomask is fabricated from a fused silica wafer and coated with chromium. The micro-cantilevers are fabricated from crystalline silicon wafers using a deep reactive ion etching (DRIE) process. A double-sided polished 10 μm thick crystalline silicon wafer is bonded to a 500 μm SiO₂ handle wafer. The crystalline silicon surface is coated with a 1.3 μm layer of AZ 5214 photoresist. The photoresist is then exposed to a soft bake followed by contact exposure with the mask. The photoresist is then developed and hard baked. The wafer is mounted on a 6″ handling substrate for DRIE. After DRIE the wafer is inspected and the photoresist removed via a wet strip followed by plasma cleaning. After etching is completed the wafer is cut into 15 mm×15 mm pieces which contain the cantilever arrays by dicing. Dicing is followed by HF release and supercritical CO₂ drying.

Detection System Setup and Measurements.

In preliminary and published studies we have already optimized the microcantilever length and thickness for producing an optimal deflection from a contracting myotube, although this can be adjusted if required. Our current configurations have proven sensitive enough to provide detailed force measurements of myotube contraction (Wilson et al. 2007) (See FIGS. 14, 8).

Alternatively, the SAM chemistry can also be modified to adjust cell/substrate adhesion. The deflection of the muscle-actuated cantilever is measured using optical detection. Actuation of the reflex arc is observed indirectly using diode laser beam bounce techniques and position sensitive detectors (i.e., standard AFM technology) as outlined in the preliminary data section of the proposal and in (Wilson et al. 2007). Initially, cantilevers are examined with SEM techniques to determine their average length, with and thickness. Subsequently, the average spring constant can be calculated by theoretical means and the Young's modulus for crystalline silicon. The spring constants are about 1.2 N/m.

Because of the large variability in the spring constants, cantilevers have to be further calibrated on an individual basis when used for precision force measurements. This variability is most likely caused by variations in thickness of the cantilever. Variability in the length and width is often quite small because typical lateral resolution in photolithography is on the submicrometer scale. For nominal spring constants greater than 0.1 N/m, we use the calibrated load-displacement transducer of a nanoindenter to measure the spring constant of each cantilever in an array. Measuring the resonance frequency of individual cantilevers and applying it to Sader's equation provides detailed spring constants. Another important value is the thickness of myotubes, located on cantilevers, in order to calculate their internal stress values. Confocal microscopy provides z-stacks of fluorescently died myotubes, which then can be analyzed for cell heights. The measurement is initially made in triplicate in order to determine the mean, standard deviation, and standard error of the sample set. The number of measurements may be increased in order to bring the range of the confidence interval to 99%.

Surface Modification and Characterization.

Trimethoxysilylpropyl-diethylenetriamine (DETA) has been demonstrated to support adhesion and growth of embryonic rat, adult rat as well as human myocytes and satellite cells. Tridecafluoro-1,1,2,2-tetrahydrooctyl-1-dimethylchlorosilane (13F) and polyethylene glycol silane (PEG) monolayers are cytophobic for this cell type. Self-assembled monolayers (SAMs) are prepared according to our published procedures (Hickman et al. 1994). In brief, silica slides are cleaned by immersion in 1:1 methanol/HCl, followed by H₂O and then concentrated H₂SO₄. Cleaned substrates are then transferred to boiling water prior to reaction with the silanes. An alternative to this procedure, which may be necessary for the silicon devices, is to clean in a non-directional O₂ plasma. In general, SAMs are formed by immersing clean silica substrates in organic solvents containing 1-2% silane, and then rinsing the slide three times with the same solvent. After the final rinsing step, the slides are baked on a hotplate to quickly remove residual solvent and to promote complete reaction of the silanes with the reactive surface groups.

SAM-modified surfaces are characterized using XPS to demonstrate formation of the SAM and contact angle measurements to quantify wettability. Contact angle measurements are a rapid and simple measure of wettability. Contact angles are measured by application of static, sessile drops (5-30 μl) of deionized water to substrate surfaces with a micropipetter. The measurements are made visually on both sides of the drops using a Rame-Hart type goniometer. XPS is a technique for the elemental analysis and characterization of surfaces (Briggs 1992). Since the electrons of each element possess characteristic binding energies, the energy pattern of emitted photoelectrons arising from a given element serves to unambiguously identify that element, while the precise peak positions, or chemical shifts, reflect the chemical environment (i.e., oxidation state) in which the element is found. XPS measurements are obtained on a FISONS 220i XL spectrometer with imaging capability to 2 μm resolution. For each sample examined by XPS, we obtain a survey spectrum and high-resolution spectra for the elements Si, C, N, and any other element that is unique to the SAM (F for 13F). These measurements serve as (a) baseline quantities against which to contrast properties of the surface after cell culture, and (b) baseline quantities against which to contrast cell growth and survival from experiment to experiment for multivariate analysis.

Surface Patterning.

If necessary, surface patterns are made using projection lithography that avoids direct contact with the surface and is easily integratable with the barriers. SAMs prove an ideal tool for the design of circuits for the study of neuronal interactions in a defined minimalistic system. Lithographic patterns on a silica substrate are prepared by exposure of a SAM-modified surface to 193 nm UV light followed by re-derivatization with another SAM. This process has been described previously, and we have used this method to create patterns for the preferred attachment of several types of cells (Stenger et al. 1992; Spargo et al. 1994; Ravenscroft et al. 1998). We already have demonstrated that myotubes will grow on DETA, but not 13F or PEG and that patterns can be made. Masks are prepared as needed for experiments; changes and alterations are straightforward. XPS image analysis allows us to determine whether the initial silane modifier is removed from the surface with laser ablation, and to verify that rederivatization of the second silane has occurred. We then culture cells on patterned surfaces to determine fidelity to the patterns. We use phase microscopy to assess which SAM combinations result in specifically placed myofibrils.

Muscle Cell Preparation.

Primary culture of rat skeletal muscle cells is obtained by the methods used by Daniels and Nelson et al., (Shainberg et al. 1976; Daniels 1990; Nelson et al. 1993). Cell suspensions are obtained by trypsinization of muscle pieces from hind limbs of newborn rats. Cells are dissociated by incubation in trypsin, followed by resuspending the cells in L15 medium supplemented with 10% fetal calf serum to inhibit trypsin activity. The cells suspension is triturated and the supernatant is collected. The supernatant is centrifuged and the pellet resuspended in the serum free media developed in our laboratory. As an additional improvisation to this technique, after centrifugation the pellet is resuspended in a serum free medium and incubated in a 90 mm tissue culture dish, which results in the settling down of the fibroblasts such that the cells in the suspension consist of a pure population of myoblasts (Kuhl et al. 1982). The suspension is obtained and centrifuged and the pellet is resuspended in the culture media and is used for plating. The cells are plated at a density of 250 cells/mm². The myocytes (250 cells/mm²) are plated in a serum free composition containing L15, Media 199, B27 supplement, GDNF, BDNF, Cardiotrophin, bFGF2, IGF, PDGF, Thyroxine. The medium is changed every 4th day. Cells can be maintained in this serum free media now for up to 3 months (Das et al. 2009).

Adult Rat Satellite Cell Culture

Primary culture of adult rat satellite cells is obtained by methods described by (Huang et al. 2005). Rat hindlimb tibialis anterior muscles are excised and minced into pieces. The tissue is dissociated by incubation in type II collagenase followed by resuspension in L15 medium supplemented with 10% fetal bovine serum to inhibit enzyme activity. The tissue suspension is triturated and then incubated in a 90 mm dish, which results in the settling down of the fibroblasts such that the cells in suspension consist of a pure population of satellite cells. The suspension is removed from the dish and centrifuged and the pellet is resuspended in culture medium followed by cell plating. The satellite cells are plated at a density of 400 cells/mm². The next day the medium is aspirated, pelleted and then replated. This is repeated the following day. This serial plating results in further enrichment of the satellite cell population (Malerba et al. 2009).

Fabrication and Characterization of the Piezoelectric Microcantilever System.

The advantage is that the mechanical device and the readout electronics can be implemented in the same integrated circuit. Replacing the optical readout with piezoelements reduces the size and complexity of the microcantilever system. Further, experimental data can be obtained from multiple cantilevers in parallel and the reverse piezoelectric effect can be used to employ the cantilevers as actuators to exercise muscle or activate intrafusal fibers enabling each cantilever to become a stand alone sensor element.

Prior to the fabrication of cantilevers, electrical circuits and piezoelectric components are applied to the silicon wafer. Subsequently, the actual cantilevers are etched exactly in those positions were piezoelectric components were placed. Individual piezoelectric cantilever chips are packaged with printed circuit boards providing up to 60 contact pads to allow the readout and control of piezoelectric cantilevers with a head stage usually used for multielectrode arrays (Multichannel Systems).

In the first part of this disclosure, we show that we can fabricate a cantilever assembly for the culture of embryonic myotobes an use it to measure contraction of the myotube by monitoring cantilever deflection using an AFM-like laser system. In this portion of the disclosure, we integrate piezoelectric elements onto the cantilevers to measure cantilever deflection.

Fabrication of a Piezoelectric Elements for Cantilever Arrays.

Silicon wafers with silicon on insulator serve as base material in the fabrication of piezoelectric cantilevers. An additional layer of 100-200 nm SiO₂ is deposited onto the base material to insulate conductive materials from the semi-conductive silicon. Subsequent fabrication steps are depicted in FIG. 16A (left). Metal layers are fabricated to connect the piezoelectric components with microelectronics. Layers of piezoelectric materials, such as ZnO and PTZ sol-gel, are deposited exactly in those areas where microcantilevers remain after the etching process. Another conductive layer contacts the piezoelectric components from top to apply voltages for actuation or current read out during sensor mode. An insulation layer of silicon-ONO-stacks (oxide-nitride-oxide) protects conductive elements from aqueous solutions during cell culture. An alternative approach is presented in FIG. 16A (right), where piezoelectric elements are replaced by piezoresistive materials. This alternative approach offers a higher sensitivity during readout, however, piezoresistive materials do not provide the usage of the cantilevers as actuators and the field stimulator would then have to be retained in the system.

Piezoelectric cantilevers are characterized using the current laser system. The sensor application for the detection of microcantilever deflection by an external force is compared to laser measurements and the actuator application of cantilevers by the inverse piezoelectric effect is monitored by the laser system as well. Representative experiments using embryonic rat cells are being repeated with the piezoelectric cantilever system and monitored in parallel by the laser system. The skilled should know that due to a different surface texture caused by additional piezoelectric layers under an insulating silicon nitride surface, small variations in the cell culture protocols may be necessary to optimize the cell attachment. Due to additional piezoelectric and insulating layers on cantilevers, differences between standard cantilevers and piezoelectric cantilevers during laser measurements are expected. Finding and quantifying these differences are part of the characterization process for piezoelectric cantilevers. Characterization of the cell morphology and surface chemistry is as described above. Experiments with the three cell types described above to verify that the surface chemistry and myotube differentiation are not altered significantly due to the change in bulk surface composition of SAM formulation after force transduction testing and confirm the systems are equivalent.

The additional layers on cantilevers are expected to cause some mechanical stress resulting in slightly bent cantilevers. The variation of pre-deflection due to intrinsic tress is expected to be close to constant across the wafer, allowing for a determination of an average pre-deflection by microscopic (SEM) means. The pre-deflection of cantilevers can be considered as an advantage. In order to deflect a straight cantilever myotubes initially need to develop intense forces, whereas slightly pre-deflected cantilevers allow for a better myotube-force to cantilever-deflection ratio. Using the reverse piezoelectric effect, a pre-deflection of cantilevers could be eliminated by applying a bias voltage. Further, measurements with piezoelectric cantilevers are expected to have a higher noise level as compared to results obtained with the laser system. However, the scalability of experiments using piezoelectric cantilevers increases the amount of data that can be obtained and thus compensates for noise by statistical means. As an alternative approach we propose the usage of piezoresistive elements (see FIG. 16). The sensitivity of piezoresistive cantilevers is expected to be in between the piezoelectric and the laser approach. Experiments based on piezoelectric cantilevers used as actuators cannot be conducted with piezoresistive cantilevers. Thus the piezoresistive approach is considered as an alternative to allow at least the minimalization of the current system. As another alternative, if the surface modification on the materials used to create piezoelectrics is not acceptable for cell growth, the skilled may want to try a number of solutions. An extra step of oxygen plasma treatments could be added to create more hydroxyl groups on the surface for the silane modification. Also, derivatization with DETA and addition of vitronectrin to the surface could be tried, as we have previously shown this promotes surface myotube formation (Molnar et al. 2007).

Conclusions

The present disclosure demonstrates the development of a novel Bio-MEMS device based on the use of microfabricated silicon microcantilevers and alkylsilane surface chemistry for the study of skeletal muscle and its development. The usefulness of this device has been demonstrated for real-time interrogation of cultured skeletal muscle and the quantification contractile stress and kinetics. It has also been shown that physiological phenomena can be monitored and quantified, and responds to exogenously applied factors.

Cultured myocytes on silicon microcantilevers coated with DETA spontaneously differentiate into functional myotubes that produce contractile stress sufficient to deflect the microcantilevers. These deflections were then measured using a laser detection system. By applying electrical field stimulation, it was possible to selectively actuate the myotubes on microcantilevers in a frequency and intensity dependent manner. This ability to selectively actuate a microcantilever is advantageous as it allows a high degree of control over the timing and nature of contraction.

This method could also be applied to create bio-robotic devices using skeletal muscle as an actuator on a microfabricated device. Previous studies have utilized cardiomyocytes to provide mechanical force. However, cardiac tissue contracts in a primarily spontaneously manner unlike skeletal muscle which remains inactive in the absence of stimulating inputs. Also, skeletal muscle is preferable over cardiac muscle due to its rate-response characteristics. As stimulation frequencies increase, contraction frequency and force generation of skeletal muscle will also increase until tetanus is induced, resulting in tonic contraction. Cardiac muscle, on the other hand, will cease to contract under high frequency stimulation, a situation similar to that of cardiac infarction.

This technique holds particular promise for applications in drug discovery and as a model for various diseases involving skeletal muscle. The development of an in vitro model for functional biological circuits would greatly benefit the broader scientific community and society in general. By creating lab-on-chip systems that allow high-throughput, real-time experimentation, research costs would be reduced, data collection and analysis would be simplified, and the need for costly and ethically questionable animal studies would be reduced.

According to the above description and in the drawings and specification there have been disclosed typical preferred embodiments of the invention and although specific terms may have been employed, the terms are used in a descriptive sense only and not for purposes of limitation. The invention has been described in considerable detail with specific reference to these illustrated embodiments. It will be apparent, however, that various modifications and changes can be made within the spirit and scope of the invention as described in the foregoing specification and as defined in the appended claims.

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TABLE 1
Comparison of calculated stress values to published literature.
Values for ds/dt are not available in Close et al., but average
force generation has been reported to be more than 1000 fold
higher that measured in Dennis and Kosnik.
	σF (kPa)	TPT (ms)	½ RT (ms)	dσ/dt (Pa/ms)
ESM	~1.1	236.8 ± 26.1	233.6 ± 23.8	7.15
Cultured	~2.9	69.3 ± 9.4	116.4 ± 19.4	75.3 ± 10.0
Myoids
Adult	>300	36.0 ± 2.3	48.0 ± 3.4	X

TABLE 2
Contractile properties of NbActiv4 cultured muscle versus previous
results and published literature. Values for ds/dt are not available
in Close et al., but average force generation has been reported to
be more than 1000 fold higher that measured in Dennis and Kosnik.
	σF (kPa)	TPT (ms)	½ RT (ms)	dσ/dt (Pa/ms)
ESM	~1.1	236.8 ± 26.1	233.6 ± 23.8	7.2
NbActiv4	~3.2	172.1 ± 4.7	175.6 ± 3.6	35.4
Cultured	~2.9	69.3 ± 9.4	116.4 ± 19.4	75.3 ± 10.0
Myoids
Adult	>300	36.0 ± 2.3	48.0 ± 3.4	X

Claims

1. A method of quantitatively measuring the physiological response to an agent, comprising:

culturing a plurality of muscle cells on a piezoelectric microcantilever;

contacting the muscle cells with the agent, the agent causing the muscle cells to generate contractile stress and deflect the piezoelectric microcantilever;

measuring the deflection of the piezoelectric microcantilever; and

correlating the measured deflection to effectiveness of the agent in causing a physiological response.

2. The method of claim 1, wherein measuring the deflection of the piezoelectric microcantilever comprises detecting a piezoelectric signal caused by the deflection of the piezoelectric microcantilever.

3. The method of claim 2, further comprising estimating a contractile stress on the piezoelectric microcantilever based on the measured deflection.

4. The method of claim 3, wherein measuring the deflection of the piezoelectric microcantilever further comprises numerically quantitating deflection of a free end of the piezoelectric microcantilever using a laser and optical sensor.

5. The method of claim 4, wherein numerically quantitating deflection of a free end of the piezoelectric microcantilever further comprises using Stoney's equation to calculate a contractile stress generated by the muscle cells.

6. The method of claim 5, further comprising characterizing contractile stress-piezoelectric signal response of the piezoelectric microcantilever based on the calculated contractile stress generated by the muscle cells.

7. The method of claim 1, wherein the piezoelectric microcantilever is pre-deflected due to intrinsic stress of the piezoelectric microcantilever.

8. The method of claim 7, wherein the piezoelectric microcantilever further comprises at least one of a conductive layer or an insulating layer, wherein the at least one of the conductive layer or the insulating layer causes the intrinsic stress.

9. The method of claim 7, further comprising applying a bias voltage to the piezoelectric microcantilever to eliminate the pre-deflection.

10. The method of claim 1, further comprising coating diethylenetriamine (DETA) on the piezoelectric microcantilever.

11. The method of claim 10, further comprising patterning the DETA on the piezoelectric microcantilever.

12. The method of claim 10, wherein the muscle cells are directly attached to at least a portion of the piezoelectric microcantilever coated with DETA.

13. The method of claim 1, wherein the muscle cells are aligned along a lengthwise extent of the piezoelectric microcantilever.

14. The method of claim 13, wherein the muscles cells form a plurality of myotubes.

15. The method of claim 14, wherein culturing a plurality of muscle cells on a piezoelectric microcantilever further comprises promoting growth of the myotubes along the lengthwise extent of the piezoelectric microcantilever.

16. The method of claim 14, wherein culturing a plurality of muscle cells on a piezoelectric microcantilever further comprises culturing the muscle cells to minimize variations of thickness of the myotubes on the piezoelectric microcantilever.

17. The method of claim 16, wherein a thickness of the myotubes on the piezoelectric microcantilever is greater than about 5 millimeters.

18. The method of claim 16, wherein the thickness of the myotubes on the piezoelectric microcantilever is less than about 15 millimeters.

19. The method of claim 1, wherein the agent comprises at least one of a metabolic inhibitor, a nutritional supplement, a therapeutic compound or composition, an investigational drug, or combinations thereof.

20. The method of claim 1, further comprising culturing the muscle cells in medium comprising creatine, cholesterol, and estrogen.